There is a flow control valve used for a variable displacement compressor as prior art of the present invention. This flow control valve controls the operating fluid during valve opening action by accurately positioning the valve body relative to the valve seat in accordance with electric current supplied to a solenoid. Pressure of the operating fluid, however, raises a hunting problem of the valve body. This will lead to an insufficient control of the operating fluid and unexpected operation of the variable displacement compressor and the like. FIG. 4 shows a full cross-sectional view of a flow control valve related to the art (for example, refer to patent reference 1 listed below). This flow control valve, for instance, modulates pressure and flow of the operating fluid used in air conditioner and the like. In the refrigerant cycle of the air conditioner and the like in which CO.sub.2 is used as operating fluid, generally the service pressure range becomes more than ten times compared with those of conventional refrigerants. Therefore a variety of problems may be induced by the operating fluid. Not only CO.sub.2 as operating fluid but also high-pressure operating fluid impose more difficulties on control of the operating fluid compared with conventional low-pressure operating fluid.
100 in FIG. 4 designates a flow control valve. The flow control valve 100 is comprised of a valve main body 101 and a solenoid portion 120. The solenoid portion 120 is integrally joined with the valve main body 101. Supplying electric current to the solenoid portion 120 actuates a solenoid rod 122 being guided by a bearing 123 in accordance with the intensity of the current. Next, the valve main body 101 forms an axially extending through hole therein. A shaft 112 is disposed in the through hole in freely movable manner. Also a sliding portion of a valve body 102 connected to the shaft 112 forms a freely slidable fit engagement to the hole. Dimension of the sliding portion is given by B. Figure upper portion of this valve body 102 defines a high-pressure valve body 102A while the solenoid portion 120 side defines a low-pressure valve body 102B. Respective dimension in diameter of the high-pressure valve body 102A and the low-pressure valve body 102B is given by D. Conical surfaces formed at the end tips of the high-pressure valve body 102A and the low-pressure valve body 102B are, respectively, defined as a first valve face 102C and a second valve face 102D.
The valve main body 101 disposes a suction port 106 which introduces fluid of suction pressure Ps, and the suction port 106 is able to communicate a control chamber (pressure regulation camber), not shown, via suction relief valve and orifice which are disposed in a communication passage, not shown. As shown in the upper portion of the figure, a second control port 105 is disposed which is able to communicate the control chamber and a second valve chamber. The second control port 105 admits fluid of control pressure Pc2. Even further up in the figure, there is disposed a first control port 104 which is able to communicate a first valve chamber 107 and the control chamber. The second control port 104 admits fluid of control pressure Pc1. The second valve chamber and suction passageway 106 communicate each other via a bypassing passageway. In the valve main body 101, a first valve seat is formed on the periphery of the first valve orifice which is located at the interface which communicates a discharge port 103 with the first valve chamber 107 in which the first valve face 102C lifting from or resting on the first valve seat makes opening/closing of the discharge port 103. And the fluid under discharge pressure Pd is allowed to flow into the first valve chamber 107 side from the discharge port 103. Also a second valve seat is formed on the periphery of the second valve orifice of a communication passage port in which the second valve face 102D lifting from or resting on the second valve seat makes opening/closing the passage between the second valve chamber and the suction port 106. The dimension A of the diameter of the discharge port 103 is identical to the dimension C of the diameter of the communication passage port.
In the flow control valve thus configured, the diameters of the first valve orifice and the second valve orifice, which lifts from or rest on the first valve face 102C and the second valve face 102D, respectively, share the same dimension. Therefore the forces exerted to the valve body 102 by the control fluid Pc1 and the control fluid Pc2 negate each other. This implies that the valve body 102 is actuated by means of suction pressure Ps and discharge pressure Pd alone. When the pressure differential between discharge pressure Pd and suction pressure Ps becomes greater than an attraction force determined by the current supplied to the solenoid portion 120, high-pressure valve body 102A opens so as to achieve flow control. In such an operation of the valve body 102, since the diametral dimension D of the high-pressure valve body 102A is greater than the diametral dimension A of the discharge port 103, a decrease in pressure differential between discharge pressure Pd and suction pressure Ps will reduce a pressure-driven retaining force of the valve body, thereby inducing a hunting phenomenon in which the valve body 102 makes pulsating movement in the axial direction because the valve body 102 is easily susceptible to a force due to pulsation or turbulent flow of the fluid under discharge pressure Pd. Occurrence of such hunting phenomenon in the valve body 102 makes it difficult to conduct a flow control. Also as the magnitude (intensity) of the current supplied to the solenoid portion 120 no longer remains proportionate to the operation speed in opening/closing of the valve body 102, a flow control for the fluid under discharge pressure Pd by means of the valve body 102 is likely to deteriorate.
Patent reference 1: Japanese Patent Laid-Open Publication No. 2003-328936 (FIG. 2 and FIG. 3)